As one of the means to improve soft ground, a large diameter jet grouting method for constructing a column-shaped improvement body having a large radial dimension in the ground is well-known.
In this large diameter jet grouting method, it is necessary to inject a jet stream with a long distance (a ground penetrating or cutting distance) to penetrate or cut into the ground which is to be subjected to construct a column-shaped improvement body having a large radial dimension. Therefore, the quality of a jet nozzle must be maintained equal or better than a certain level. When a poor-quality jet nozzle is used, a jet stream reachable distance, i.e., the ground penetrating distance is shortened.
In order to avoid a situation that the jet stream reachable distance is shortened, it is necessary to examine a jet stream ejected from a jet nozzle at a stage before actually injecting the jet stream in a construction site carrying out the large diameter jet grouting method, and to determine whether the jet nozzle is good or defective.
However, there is no technology to correctly determine whether a jet nozzle is good or defective at the stage before actually injecting a jet stream in a construction site.
In a prior art, for example, a jet stream is caused to impact pressure sensitive paper, a degree of discoloration of the pressure sensitive paper after impacting the jet stream is observed, and then, it is possible to determine whether a jet nozzle is good or defective (see Patent Literature 1).
However, since the pressure sensitive paper discolors when a prescribed pressure is applied thereto, it is not possible to identify a point of time at which the prescribed pressure (or a higher pressure which causes discoloration) was applied even though the pressure sensitive paper discolored. Further, since the point of time at which the predetermined pressure was applied cannot be identified, there is a problem that even a defective nozzle, by which a pressure of a jet stream cannot act always uniformly, is determined to be a non-defective product.
As another prior art, a technology to examine whether an injection hole in a cleaning device which ejects a cleaning liquid with a high pressure is clogged has been proposed (see Patent Literature 2).
However, the cleaning device according to the prior art (Patent Literature 2) cannot be applied to ground improvement construction works.